In the prior art, various methods for dissolving cellulose are described. A few publications describe enzymatic treatments in connection with processes for dissolving cellulose.
Publication WO 01/96402 discloses a method in which an enzymatically treated cellulose is dissolved in an aqueous solution of alkali metal hydroxide having a concentration which is not lower than 5 wt.-%. The publication states that the alkaline solutions are stable at temperatures above 0° C. Urea and zinc oxide are applied for the improvement of stability and quality of solutions. The highest reported cellulose concentration in the alkaline solution is 5.2% (NaOH concentration of the alkaline solution containing cellulosic material is 8.57%, see Example 6).
Publication FI 107335 discloses a method in which an enzymatically treated cellulose is dissolved in an aqueous solution of alkali metal hydroxide having a concentration which is from 5 to 15 wt.-%. The publication states that the cellulose concentration may be even 8 wt.-%. However, the following concentrations are reported in the examples of the publication:
Cellulose concentrationThe concentration(wt.-% in the alkalineof the alkalineExamplehydroxide solution)hydroxide solution in wt.-%13.3923.3734.81046.9953.3965974.5984.59
Examples 9 to 14 do not contain information on the above-mentioned concentrations. It is reported in examples 1 to 14 that the solubility is quite high. However, the used method is purely visual, and therefore, it is possible that the solution contains insoluble cellulose although it is not detected by the visual method.
The problem with the methods of prior art is that the cellulose content cannot be high if good solubility is expected when the aqueous solution in which the cellulose is dissolved comprises alkali metal hydroxide between 3.5 wt.-% and 7 wt.-%.